This invention relates to fuel cells and, in particular, to replenishment of electrolyte in molten carbonate fuel cells.
A fuel cell is a device that directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte, which serves to conduct electrically charged ions. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive separator plate between each cell.
Molten carbonate fuel cells (MCFCs) operate by passing a reactant fuel gas through the anode, while oxidizing gas is passed through the cathode. The anode and the cathode of MCFCs are isolated from one another by a porous electrolyte matrix, which may comprise a plurality of layers saturated with carbonate electrolyte. Typical MCFC designs include carbonate electrolyte stored in non-electrolyte matrix components such as the pores of the anode and cathode and in gas passages formed in the anode and cathode current collectors or the bipolar separator plates. The carbonate electrolyte melts during the initial heat up of the fuel cell and redistributes among the pores of the anode, the cathode and the electrolyte matrix due to the capillary forces of the pores. The maximum amount of electrolyte inventory allowed in the beginning of fuel cell life is limited because excess electrolyte in the fuel cell results in poor cell performance due to electrolyte flooding of the anode and the cathode pores.
During MCFC operation, the electrolyte in the cells is consumed by corrosion and lithiation reactions with the cell components and through evaporation. Electrolyte loss in the cells may increase electrode polarization and the ohmic resistive loss across the electrolyte matrix. Moreover, drying of the electrolyte matrix eventually leads to gas cross-over and cell failure.
FIG. 1 shows a graph of relative electrolyte inventory in a MCFC system at various operating times. As shown in FIG. 1, a significant amount, i.e. about 10%, of electrolyte is consumed during the first 2,000 hours of MCFC operation. This occurs due to surface corrosion of the fuel cell components, particularly of the cathode current collector and the bipolar separator plate. The electrolyte loss during the first 2,000 hours of operation, unless replenished, results in a 30%, or about 20,000 hour, reduction in cell life. Accordingly, as can be seen from FIG. 1, the serviceable lifetime of MCFCs is limited by the electrolyte inventory of the cells.
Replenishing of the electrolyte in the MCFCs after 2000 hours of operation can increase the operating life of the fuel cell system by approximately 2 years. As a result, various methods of providing additional electrolyte to the matrix and of replenishing electrolyte in the cells internally and externally have been developed. For example, U.S. Pat. No. 5,468,573 discloses a method of providing additional electrolyte to the matrix internally by packing the current collector recesses with electrolyte paste consisting of 70% carbonate powder and 30% carrier vehicle. When the system of the '573 patent is heated to operating temperatures, the electrolyte paste in the current collector decomposes by driving off the carrier and melted carbonate electrolyte is absorbed into the electrolyte matrix and electrode plates.
Another method of providing additional electrolyte and also of supplementing MCFCs with electrolyte is disclosed in U.S. Pat. No. 5,563,003. The '003 patent teaches loading a plurality of sealed soluble containers filled with electrolyte in the corrugated separator plate of the MCFC. These sealed containers are formed so as to dissolve from their inner walls to release supplementary electrolyte into the cell during fuel cell operation at approximately the time when the electrolyte in the cell becomes deficient.
Another technique for adding electrolyte to the matrix of a carbonate fuel cell is disclosed in Japanese Patent Application Laid Open Number 01-183069. In this technique, the electrolyte matrix includes four matrix layers and multiple carbonate layers situated between the matrix layers. A first carbonate layer having a melting point of 489° C. is situated between the second and third matrix layers and a second carbonate layer having a melting point of about 600° C. is inserted between the third and fourth matrix layers. The fuel cell temperature is raised to the melting point of the first carbonate layer and this layer melts into the matrix layers with the cell being operated at a first operating temperature of 550° C. When the performance of the fuel cell decreases due to consumption of the electrolyte, the cell operating temperature is raised to 600° C. so that the second carbonate electrolyte layer melts into the matrix layers.
Although some of these methods have been helpful in maintaining sufficient electrolyte levels in MCFCs, the methods are difficult to implement, require multiple carbonate layers and multiple operating temperatures and additionally require operational downtime for servicing, all of which increase system manufacturing costs. In particular, the use of small stainless steel electrolyte storage containers in the fuel cell system to provide supplemental electrolyte to the cells, as disclosed in the '003 patent, results in increased material and manufacturing costs. Moreover, the method described in the '573 patent provides additional electrolyte to the system during the start-up of the system without delaying the addition of electrolyte until after some of the electrolyte inventory is lost during operation. Finally, the technique of '069 Japanese published patent application requires multiple carbonate layers in the electrolyte matrix and changes in the operating temperature of the fuel cell.
It is therefore an object of the present invention to provide an apparatus and method for delayed addition of electrolyte to a fuel cell to replace electrolyte lost during the fuel cell operation.
It is a further object of the present invention to provide an apparatus and method adapted to provide additional electrolyte to a fuel cell, which is easy to implement and does not result in significant cost increases.